37 research outputs found
A tunable coupling scheme for implementing high-fidelity two-qubit gates
The prospect of computational hardware with quantum advantage relies
critically on the quality of quantum gate operations. Imperfect two-qubit gates
is a major bottleneck for achieving scalable quantum information processors.
Here, we propose a generalizable and extensible scheme for a two-qubit coupler
switch that controls the qubit-qubit coupling by modulating the coupler
frequency. Two-qubit gate operations can be implemented by operating the
coupler in the dispersive regime, which is non-invasive to the qubit states. We
investigate the performance of the scheme by simulating a universal two-qubit
gate on a superconducting quantum circuit, and find that errors from known
parasitic effects are strongly suppressed. The scheme is compatible with
existing high-coherence hardware, thereby promising a higher gate fidelity with
current technologies
Two-qubit spectroscopy of spatiotemporally correlated quantum noise in superconducting qubits
Noise that exhibits significant temporal and spatial correlations across
multiple qubits can be especially harmful to both fault-tolerant quantum
computation and quantum-enhanced metrology. However, a complete spectral
characterization of the noise environment of even a two-qubit system has not
been reported thus far. We propose and experimentally validate a protocol for
two-qubit dephasing noise spectroscopy based on continuous control modulation.
By combining ideas from spin-locking relaxometry with a statistically motivated
robust estimation approach, our protocol allows for the simultaneous
reconstruction of all the single-qubit and two-qubit cross-correlation spectra,
including access to their distinctive non-classical features. Only single-qubit
control manipulations and state-tomography measurements are employed, with no
need for entangled-state preparation or readout of two-qubit observables. While
our experimental validation uses two superconducting qubits coupled to a shared
engineered noise source, our methodology is portable to a variety of
dephasing-dominated qubit architectures. By pushing quantum noise spectroscopy
beyond the single-qubit setting, our work paves the way to characterizing
spatiotemporal correlations in both engineered and naturally occurring noise
environments.Comment: total: 22 pages, 7 figures; main: 13 pages, 6 figures, supplementary:
6 pages, 1 figure; references: 3 page
Characterizing and optimizing qubit coherence based on SQUID geometry
The dominant source of decoherence in contemporary frequency-tunable
superconducting qubits is 1/ flux noise. To understand its origin and find
ways to minimize its impact, we systematically study flux noise amplitudes in
more than 50 flux qubits with varied SQUID geometry parameters and compare our
results to a microscopic model of magnetic spin defects located at the
interfaces surrounding the SQUID loops. Our data are in agreement with an
extension of the previously proposed model, based on numerical simulations of
the current distribution in the investigated SQUIDs. Our results and detailed
model provide a guide for minimizing the flux noise susceptibility in future
circuits.Comment: 14 pages, 6 figure
Realization of high-fidelity CZ and ZZ-free iSWAP gates with a tunable coupler
High-fidelity two-qubit gates at scale are a key requirement to realize the
full promise of quantum computation and simulation. The advent and use of
coupler elements to tunably control two-qubit interactions has improved
operational fidelity in many-qubit systems by reducing parasitic coupling and
frequency crowding issues. Nonetheless, two-qubit gate errors still limit the
capability of near-term quantum applications. The reason, in part, is the
existing framework for tunable couplers based on the dispersive approximation
does not fully incorporate three-body multi-level dynamics, which is essential
for addressing coherent leakage to the coupler and parasitic longitudinal
() interactions during two-qubit gates. Here, we present a systematic
approach that goes beyond the dispersive approximation to exploit the
engineered level structure of the coupler and optimize its control. Using this
approach, we experimentally demonstrate CZ and -free iSWAP gates with
two-qubit interaction fidelities of % and %,
respectively, which are close to their limits.Comment: 28 pages, 32 figure
Broadband Squeezed Microwaves and Amplification with a Josephson Traveling-Wave Parametric Amplifier
Squeezing of the electromagnetic vacuum is an essential metrological
technique used to reduce quantum noise in applications spanning gravitational
wave detection, biological microscopy, and quantum information science. In
superconducting circuits, the resonator-based Josephson-junction parametric
amplifiers conventionally used to generate squeezed microwaves are constrained
by a narrow bandwidth and low dynamic range. In this work, we develop a
dual-pump, broadband Josephson traveling-wave parametric amplifier that
combines a phase-sensitive extinction ratio of 56 dB with single-mode squeezing
on par with the best resonator-based squeezers. We also demonstrate two-mode
squeezing at microwave frequencies with bandwidth in the gigahertz range that
is almost two orders of magnitude wider than that of contemporary
resonator-based squeezers. Our amplifier is capable of simultaneously creating
entangled microwave photon pairs with large frequency separation, with
potential applications including high-fidelity qubit readout, quantum
illumination and teleportation
High-Fidelity, Frequency-Flexible Two-Qubit Fluxonium Gates with a Transmon Coupler
We propose and demonstrate an architecture for fluxonium-fluxonium two-qubit
gates mediated by transmon couplers (FTF, for fluxonium-transmon-fluxonium).
Relative to architectures that exclusively rely on a direct coupling between
fluxonium qubits, FTF enables stronger couplings for gates using
non-computational states while simultaneously suppressing the static
controlled-phase entangling rate () down to kHz levels, all without
requiring strict parameter matching. Here we implement FTF with a flux-tunable
transmon coupler and demonstrate a microwave-activated controlled-Z (CZ) gate
whose operation frequency can be tuned over a 2 GHz range, adding frequency
allocation freedom for FTF's in larger systems. Across this range,
state-of-the-art CZ gate fidelities were observed over many bias points and
reproduced across the two devices characterized in this work. After optimizing
both the operation frequency and the gate duration, we achieved peak CZ
fidelities in the 99.85-99.9\% range. Finally, we implemented model-free
reinforcement learning of the pulse parameters to boost the mean gate fidelity
up to , averaged over roughly an hour between scheduled
training runs. Beyond the microwave-activated CZ gate we present here, FTF can
be applied to a variety of other fluxonium gate schemes to improve gate
fidelities and passively reduce unwanted interactions.Comment: 23 pages, 16 figure